Color Continuum

Hola podrian ayudarme con la frase "color continuum" en el siguiente contexto: Mexico's racial dynamics are perhaps best summed up by Steve Sailer in his article, "Where Did Mexico's Blacks Go?" He writes that "[w]hat Mexico does have instead of a color line is a 'color continuum.' There are no sharp racial divides, yet the rule for social prestige remains 'the whiter the better.'"